Toilet gurgles

When the upstairs shower is running, and sometimes when that toilet is flushed, the lower toilet gurgles. I am trying to reason which is more likely the issue; vent closing in, or drain? It seems I have a venting issue, perhaps ice is building up in the vent pipe, causing the line to gulp air in fits and starts, leading to gurgling. I've had slight issues with the vent before, and we've had a lot of -10 to -15. Re-insulating the vent pipe was on my list this summer, but....